A secondary battery generally includes a CID (Current Interrupt Device) in case of excessive increase in internal pressure.
There is a known current interrupt device which includes a reverse plate acting as a diaphragm and a holder as an insulator for holding the reverse plate, the reverse plate being welded to a collector tab of a positive electrode at a predetermined welding strength.
In such a current interrupt device, when internal pressure of a secondary battery increases, the reverse plate pushed by electrolyte solution is reversed and separated from the collector tab (conduction is blocked) to interrupt electric current. For this purpose, in the current interrupt device with this structure, the welding strength between the collector tab and the reverse plate is set to a value lower than joint strengths between other portions.
Therefore, a welded portion between the collector tab and the reverse plate in the current interrupt device is broken more easily than the other portions.
As a secondary battery including a current interrupt device, there is a publicly known secondary battery disclosed in JP-A 2011-243559 described below, for example.
The conventional secondary battery shown in JP-A 2011-243559 includes a current interrupt device which has a reverse plate acting as a diaphragm and connected to a rivet, and a holder as an insulator for holding a collector tab of a positive electrode, and which is formed by welding the reverse plate to the collector tab of the positive electrode at a predetermined welding strength. In the conventional secondary battery, a positive electrode terminal is placed on the upper face of a sealing plate, and the holder is placed on the lower face of the sealing plate. In addition, the positive electrode terminal and the holder are joined to the sealing plate by the rivet, and thereby the positive electrode terminal and the collector tab of the positive electrode are electrically connected via the rivet and the reverse plate.
FIGS. 7 and 8 show a structure around the positive electrode in the conventional secondary battery.
As shown in FIG. 7, a positive electrode 50 in the conventional secondary battery includes a current interrupt device 45 which has a reverse plate 41 acting as a diaphragm and connected to a rivet 48 and a holder 42 as the insulator for holding a collector tab 44 of the positive electrode 50, and which is formed by welding the reverse plate 41 to the collector tab 44 in a welded portion 43 at a predetermined welding strength. A positive electrode terminal 47 is placed on one face (upper face in FIG. 7) of a sealing plate 46, and the holder 42 is placed on the other face (lower face in FIG. 7) of the sealing plate 46. The positive electrode terminal 47 and the holder 42 are joined to the sealing plate 46 by caulking the rivet 48, and thereby the positive electrode terminal 47 and the collector tab 44 are electrically connected via the rivet 48 and the reverse plate 41. Between the sealing plate 46 and the positive electrode terminal 47, an insulator 21 and a gasket 22 are provided.
As shown in FIG. 8, in the positive electrode 50, the holder 42 is fitted in a recess 46a formed on the lower face of the sealing plate 46, and a gap is formed between a peripheral edge of the recess 46a and the holder 42. In the conventional positive electrode 50, the holder 42 is merely joined by the rivet while fitted in the recess 46a. 
As shown in FIG. 8, in the conventional secondary battery shown in JP-A 2011-243559, since the holder 42 is fitted in the recess 46a of the sealing plate 46 to join the holder 42 and the sealing plate 46 together in the positive electrode 50, there is the gap between the holder 42 and the peripheral edge of the recess 46a of the sealing plate 46 in the fitted portion.
If there is the gap between the holder 42 and the sealing plate 46, vibration may be caused by displacement of the holder 42 and the sealing plate 46 with respect to each other and collisions between them. The vibration is transmitted to the current interrupt device 45, which may damage the welded portion 43 between the reverse plate 41 and the collector tab 44.
If there is damage such as a crack in the welded portion 43, the current interrupt device 45 is actuated at lower internal pressure. Therefore, the damage to the welded portion 43 may cause a malfunction of the current interrupt device 45.